1. Field of the Invention
The present invention relates generally to heat exchangers and methods of constructing heat exchangers.
2. Discussion of the Background
Heat exchangers and heat exchange chemical reactors having large arrays of parallel tubes are known in the art. Traditional design practices for such articles are codified in design standards. U.S. Pat. No. 6,497,856 (the '856 patent), which is hereby incorporated by reference, teaches a heat exchange chemical reactor for producing hydrogen from natural gas, propane, liquefied petroleum gas (LPG), alcohols, naphtha and other hydrocarbon fuels. Typical industrial applications include feedstock for ammonia synthesis and other chemical processes, in the metals processing industry, for semiconductor manufacture and in other industrial applications, petroleum desulfurization, and hydrogen production for the merchant gas market. The demand for low-cost hydrogen at a smaller scale than produced by traditional industrial hydrogen generators has created a market for small-scale hydrogen production apparatus (<15,000 standard cubic feet per hour (scfh)). This demand has been augmented by the growing enthusiasm for hydrogen as a fuel for stationary and mobile powerplants, especially those employing electrochemical fuel cells, which require hydrogen as a fuel.
U.S. application Ser. No. 10/436,060 (the '060 application), filed on May 13, 2003, which is incorporated herein by reference, discloses an advantageous heat exchange apparatus that provides a cost-effective heat exchange structure that reduces shell-side fluid leakage and bypass for tubular heat exchangers such as those operated at high temperatures and pressures. FIG. 1 of the '060 application shows a tubular heat exchanger core including an array of tubes 2, which are sealingly connected between a first tubesheet 3 and a second tubesheet 4. A first fluid flows from an inlet manifold sealingly attached to the first tubesheet 3, through tubes of the array of tubes 2, and out a second manifold attached to the second tubesheet 4. The array of tubes 2 is provided on outer surfaces of the tubes with flow directing baffles or plates 5, which are used to cause a second fluid to flow substantially normal to the axis of the array of tubes 2. All of the baffles have a small extended portion 18, which extends outside the flow passageways and finned zones in each fluid stage. The extended portions 18 are provided for mating to refractory ductwork for directing the flow of the second fluid. FIG. 2 of the '060 application shows a structure that provides improved manifolding of the flow within a housing 100 formed by housing members, such as sheet cover pans 20, 30 and portions of various baffles that form part of the outer shell of the heat exchanger, such as portions of baffles 13-16 and 19. The housing 100 can achieve a condition of zero leakage.
However, the inventors of the present invention have determined that the heat exchange apparatus described in the '060 application has certain capacity restrictions that are improved in the present invention. Thermal stress management is one of the largest, if not the largest, limiting factor in the reformer technology described in the '856 patent and the '060 application. Since the reformers tend to operate under high thermal stress, as the reactor is scaled up in size, a high pressure drop (i.e., change in pressure, ΔP) across the tube array can put large stresses on the baffles and the pan ductwork. These large stresses lead to premature failure due to creep at services temperatures. The pressure drop can be lowered by simply increasing the cross sectional area of the heat exchanger stages with attendant larger pan areas, however, the stresses are far greater in larger pans for the same pressure load. Thus, simply increasing the heat exchanger stage area does not provide an adequate solution. Additionally, when the reactor is scaled up in size, the overhanging burner box is plagued by high stresses, due to the large size of the pans and due to the cantilever forces from the burner. Furthermore, very big reactors require very thick tubesheets. These thick, beefy tubesheets are not only expensive, but they are also very rigid. Thus, large offset holes are required in the tubesheets in order to prevent the thermal expansion of the tubesheets from damaging the array of tubes extending therethrough, although such holes can be minimized as discussed in U.S. Pub. No. 2003/0173062A1, which is hereby incorporated in its entirety by reference. Such large through-holes limit the effectiveness of the reformer by causing bypassing of the tube arrays.
It is therefore desirable to provide a heat exchange structure that overcomes the capacity restrictions discussed above.
In the manufacture of hydrogen, and especially in the manufacture of hydrogen according to the process of U.S. Pat. No. 6,623,719 (the '719 patent) wherein the combustion air is preheated in the cooling of the water gas shift process, the simultaneous control of the flame temperature, water gas shift process temperature and steam reformer inlet temperatures can be extremely difficult. Departure from the preferred temperature conditions can cause poor fuel conversion, high thermal stresses, excessive corrosion, and problems with local condensing and reboiling of steam within the system. These deficiencies are particularly problematic during transient operation, such as startup, shutdown and load changes. It is therefore desirable to provide apparatus for and a method of controlling undesirable departures from the preferred operating temperatures.
In the '719 patent, some thermal energy is lost to the ambient as waste heat after the water gas shift process in the process condenser. This wasted heat energy undesirably increases the operating cost of the hydrogen process and increases emissions of climate change gases. It is therefore desirable to provide apparatus for and a method of recovering additional waste heat that is economical to build and does not adversely impact the operability of the hydrogen generating process.